The 3rd Generation Partnership Project (3GPP), an industry consortium that defines and promulgates wireless communication network technical specifications, has defined a number of standards, such as High Speed Packet Access (HPSA). 3GPP has introduced multi-carrier operation for the uplink in HSPA. Release 9 of the standard allows for the transmission of two signals, each modulated onto adjacent 5-Mhz uplink carrier frequencies (referred to herein as “carriers”) in parallel from the same User Equipment (UE) towards a serving base station (Node-B or eNode-B). Multi-carrier uplink has the potential to substantially increase both user throughput and system throughput. The two-carrier specification is known as Dual-Cell High Speed Uplink Packet Access (DC-HSUPA) operation. Since the carriers are adjacent in the same frequency band, it is technically feasible to transmit signal modulated onto both carriers using a single multi-carrier power amplifier (PA) with twice the bandwidth, which may provide a more economical solution than two parallel, single-carrier PAs. Under normal conditions, the two carriers operate independently from each other; for example each carrier has its own mechanisms for power control, serving grants, E-TFC selection and HARQ retransmissions.
The potential benefits of uplink transmit (Tx) diversity in the context of High-Speed Uplink Packet Access (HSUPA) is being evaluated (see, e.g., 3GPP Tdoc RP-090987, 3GPP Work Item Description: Uplink Tx Diversity for HSPA). With uplink transmit diversity, UEs that are equipped with two or more transmit antennas are capable of utilizing all of them. This is achieved by multiplying the uplink signal s(t) with a set of complex weights wi, and transmitting the signal from two or more antennas, as depicted in FIG. 1. Note that i=1 . . . N where N denotes the number of transmit antennas. In general, each path from a transmitting antenna a1 . . . aN to a receiving antenna b1 . . . bm will experience different channel characteristics hn,m across the air interface.
The rationale behind uplink transmit diversity is to adapt the weights wi so that the user and network performance is maximized. Depending on UE implementation, the antenna weights wi may be associated with different constraints. Within 3GPP two classes are considered: beam forming and switched antenna diversity.
Beam forming is a technique whereby, at any given point in time, the UE may transmit from more than one antenna simultaneously. The name beam forming is derived from the technique of controlling the phase and amplitude of signals transmitted from different antennas to create a pattern of constructive and destructive interference in the wavefront, effectively altering the gain in desired spatial directions. Because differentiated signals are transmitted from different antennas, beam forming generally requires a separate PA to drive each antenna.
In switched antenna diversity, the UE at any given point in time transmits from only one of the antennas. Thus, referring again to FIG. 1, if wi≠0, then wj=0 for all j≠i. With switched antenna diversity, a UE may use a single PA, and simply switch its output to the selected antenna. Switched antenna diversity can be considered as a special case of beam forming, where the weight of one antenna is 1 (i.e., switched on) and all others are 0 (i.e., switched off).
In either case, decisions of the use of Tx diversity and the selection of the antenna weights can be taken by the network or the UE.
The Node-B may provide explicit feedback to the UE stating whether Tx diversity should be applied, and if so which weights the UE should use when transmitting the signal. This case requires feedback, and may require the introduction of a new feedback channel.
Alternatively, the UE may autonomously decide whether Tx diversity should be applied. To decide this, the UE may monitor existing indicators and measurements of channel quality, such as the number of required HARO retransmissions, the downlink channel quality indicator (COI), or the UE transmission power headroom (UPH) measurement. For example, if the COI and/or the UPH become low, the UE may conclude that it is in a bad coverage area and that it is likely to be beneficial to activate Tx diversity.
Additionally, the UE may autonomously decide the antenna weights wi. To select the weights wi the UE may monitor existing feedback channels, including those that are transmitted for other purposes, such as the Fractional-Dedicated Physical Control Channel (F-DPCH). For example, if the UE receives a large number of consecutive Transmission Power Control (TPC) UP commands on F-DPCH from the serving cell, the UE may conclude that it is likely to be beneficial to switch to the other Tx antenna by changing the antenna weights wi.
The fundamental goal of uplink transmit diversity is to exploit the variations in the effective channel. As used herein, the term effective channel incorporates the effects of the transmit antenna(s), transmit antenna weights, and the receiving antenna(s), as well as the wireless channel between transmitting and receiving antennas. In switched antenna diversity, and possibly also in beam forming, the antenna weights wi are changed abruptly (e.g., if a UE transmitting on antenna 1 switches wholly or significantly to transmitting on antenna 2, then w1,w2 will change from (nearly) 1,0 to 0,1. Consequently, the effective channel as perceived by the receiving Node-B may change abruptly.
In prior art systems, it has been possible to base WCDMA/HSPA functionality on the assumption that the UEs always are transmitting from one antenna. Consequently, the channel and interference estimates are typically based on a filtered version of the instantaneous channel and interference measured by the Node-B. Hence, whenever the UE changes its antenna weights—and especially if the change is abrupt such as in switched antenna diversity—the channel estimates used by the receiver becomes inaccurate. This reduces the receiver performance. Additionally, uplink transmit diversity results in a discontinuity in received power at the Node-B and a transient that upsets loop power control.
The power discontinuity occurs at the Node-B whenever the effective channel between UE and Node-B changes. This discontinuity will be a combined effect of the facts that the wireless channels between the transmit antennas and receive antenna(s) are different, and that the antenna gains of the two transmit antennas are different.
An antenna switch (or rapid change in the antenna weights) also results in a transient period until the inner-loop power control (ILPC) and outer-loop power control (OLPC) have settled. If the Node-B was aware that the power discontinuity was caused by an abrupt change in antenna weights, it could adjust its behavior thereafter; for example, by freezing the OLPC.
Thus, for numerous reasons, unnecessary antenna switching should be avoided in traditional Tx diversity. Multi-carrier operation introduces additional difficulties in Tx diversity decisions.
One basis for Tx diversity algorithms is available channel quality information, such as TPC UP/DOWN commands transmitted over F-DPCH from the serving Node-B. However, when uplink Tx diversity is combined with uplink multi-carrier operation (e.g., DC-HSUPA) it is not at all clear how to adapt the single-carrier Tx diversity UE algorithm to multi-carrier operation.
For example, if the UE receives a large number of TPC UP commands for a subset of the uplink carriers, and at the same time receives TPC DOWN for some other carrier(s), it is not obvious whether, and in such case how, the UE should prioritize between the TPC commands associated with the different carriers. This scenario is realistic because different uplink carriers may be associated with different conditions with respect to, e.g., radio propagation, traffic load, interference, and the like. Accordingly, multi-carrier operation introduces significant complexity into, and renders unworkable many prior art algorithms implementing, uplink Tx diversity.